Computer Networks I: Real-Time Protocols and Multimedia Networking - Prof. Patrick Traynor, Study notes of Computer Systems Networking and Telecommunications

Download Computer Networks I: Real-Time Protocols and Multimedia Networking - Prof. Patrick Traynor and more Study notes Computer Systems Networking and Telecommunications in PDF only on Docsity! CS 3251 - Computer Networks I Georgia Tech CS 3251- Computer Networks: Quality of Service Professor Patrick Traynor 4/21/09 Lecture 28 CS 3251 - Computer Networks I Georgia TechAnnouncements • All assignment/project drop-boxes are now closed. ‣ If you have late work, it’s now too late. • This is our last lecture with new material for the semester. ‣ Interested in learning more? There’s so much more to be done! ‣ Next time, we’ll review for the final. • Details available then... 2 CS 3251 - Computer Networks I Georgia TechReal-Time Protocol (RTP) • RTP specifies packet structure for packets carrying audio, video data ‣ RFC 3550 • RTP packet provides ‣ payload type identification ‣ packet sequence numbering ‣ time stamping • RTP runs in end systems • RTP packets encapsulated in UDP segments • interoperability: if two Internet phone applications run RTP, then they may be able to work together 5 CS 3251 - Computer Networks I Georgia TechRTP Runs on Top of UDP • RTP libraries provide transport-layer interface that extends UDP: ‣ port numbers, IP addresses ‣ payload type identification ‣ packet sequence numbering ‣ time-stamping 6 CS 3251 - Computer Networks I Georgia TechRTP Example • consider sending 64 kbps PCM-encoded voice over RTP. • application collects encoded data in chunks, e.g., every 20 msec = 160 bytes in a chunk. • audio chunk + RTP header form RTP packet, which is encapsulated in UDP segment • RTP header indicates type of audio encoding in each packet ‣ sender can change encoding during conference. • RTP header also contains sequence numbers, timestamps. 7 CS 3251 - Computer Networks I Georgia TechRTP Header (2) • Timestamp field (32 bytes long): sampling instant of first byte in this RTP data packet ‣ for audio, timestamp clock typically increments by one for each sampling period (for example, each 125 usecs for 8 KHz sampling clock) ‣ if application generates chunks of 160 encoded samples, then timestamp increases by 160 for each RTP packet when source is active. Timestamp clock continues to increase at constant rate when source is inactive. • SSRC field (32 bits long): identifies source of an RTP stream. Each stream in RTP session should have distinct SSRC. 10 CS 3251 - Computer Networks I Georgia TechReal-Time Control Protocol (RTCP) • works in conjunction with RTP. • each participant in RTP session periodically transmits RTCP control packets to all other participants. • each RTCP packet contains sender and/or receiver reports • report statistics useful to application: # packets sent, # packets lost, interarrival jitter, etc. • feedback can be used to control performance ‣ sender may modify its transmissions based on feedback 11 CS 3251 - Computer Networks I Georgia TechRTCP (cont) • each RTP session: typically a single multicast address; all RTP / RTCP packets belonging to session use multicast address. • RTP, RTCP packets distinguished from each other via distinct port numbers. • to limit traffic, each participant reduces RTCP traffic as number of conference participants increases 12 CS 3251 - Computer Networks I Georgia TechRTCP Bandwidth Scaling • RTCP attempts to limit its traffic to 5% of session bandwidth. • Example ‣ Suppose one sender, sending video at 2 Mbps. Then RTCP attempts to limit its traffic to 100 Kbps. ‣ RTCP gives 75% of rate to receivers; remaining 25% to sender ‣ 75 kbps is equally shared among receivers: • with R receivers, each receiver gets to send RTCP traffic at 75/R kbps. ‣ sender gets to send RTCP traffic at 25 kbps. ‣ participant determines RTCP packet transmission period by calculating avg RTCP packet size (across entire session) and dividing by allocated rate 15 CS 3251 - Computer Networks I Georgia TechChanging the Model of Phone Calls 16 CS 3251 - Computer Networks I Georgia TechChanging the Model of Phone Calls • When you make a phone call, what exactly is it that you are trying to establish a connection with? 16 CS 3251 - Computer Networks I Georgia TechChanging the Model of Phone Calls • When you make a phone call, what exactly is it that you are trying to establish a connection with? ‣ Landlines connect you to a specific location. Once you are there, you may still have to sort out who it is you want to talk to. It’s about calling a place. ‣ Mobile Phones allow you to reach me as I move, but only if I have a device. It’s about calling a specific device. ‣ Can we change the model of telephony? What if I want to call you no matter where you are and on any number of potential devices (laptop, cell phone, landline and more)? Can telephony be about calling a person? 16 CS 3251 - Computer Networks I Georgia TechSIP • Session Initiation Protocol • Comes from IETF SIP long-term vision • All telephone calls and video conference calls take place over the Internet • People are identified by names or e-mail addresses, rather than by phone numbers. • You can reach the callee, no matter where the callee roams, no matter what IP device the callee is currently using. 17 CS 3251 - Computer Networks I Georgia TechSIP Services • Setting up a call ‣ Provides mechanisms for caller to let callee know she wants to establish a call ‣ Provides mechanisms so that caller and callee can agree on media type and encoding. ‣ Provides mechanisms to end call. • Determine current IP address of callee. ‣ Maps mnemonic identifier to current IP address • Call management ‣ Add new media streams during call ‣ Change encoding during call ‣ Invite others ‣ Transfer and hold calls 18 CS 3251 - Computer Networks I Georgia TechName translation and user location • Caller wants to call callee, but only has callee’s name or e-mail address. • Need to get IP address of callee’s current host: ‣ user moves around ‣ DHCP protocol ‣ user has different IP devices (PC, PDA, car device) • Result can be based on: ‣ time of day (work, home) ‣ caller (don’t want boss to call you at home) ‣ status of callee (calls sent to voicemail when callee is already talking to someone) Service provided by SIP servers: • SIP registrar server • SIP proxy server 21 CS 3251 - Computer Networks I Georgia TechSIP Registrar • When Bob starts SIP client, client sends SIP REGISTER message to Bob’s registrar server ‣ similar function needed by Instant Messaging 22 REGISTER sip:domain.com SIP/2.0 Via: SIP/2.0/UDP 193.64.210.89 From: sip:bob@domain.com To: sip:bob@domain.com Expires: 3600 Register Message: CS 3251 - Computer Networks I Georgia TechSIP Proxy • Alice sends invite message to her proxy server ‣ contains address sip:bob@domain.com • proxy responsible for routing SIP messages to callee ‣ possibly through multiple proxies. • callee sends response back through the same set of proxies. • proxy returns SIP response message to Alice ‣ contains Bob’s IP address • proxy analogous to local DNS server 23 CS 3251 - Computer Networks I Georgia TechChapter 7 outline • 7.1 Multimedia Networking Applications • 7.2 Streaming stored audio and video • 7.3 Making the best out of best-effort service • 7.4 Protocols for Real-Time Interactive Applications ‣ RTP, ‣ RTCP, ‣ SIP • 7.5 Providing multiple classes of service • 7.6 Providing QoS Guarantees 26 CS 3251 - Computer Networks I Georgia TechProviding Multiple Classes of Service • thus far: making the best of best effort service ‣ one-size fits all service model • alternative: multiple classes of service ‣ partition traffic into classes ‣ network treats different classes of traffic differently (analogy: VIP service vs regular service) • granularity: differential service among multiple classes, not among individual connections • history: ToS bits 27 01 CS 3251 - Computer Networks I Georgia TechMultiple Classes of Service: Scenario 28 R1 R2 H1 H2 H3 H4 1.5 Mbps linkR1 output interface queue CS 3251 - Computer Networks I Georgia TechPrinciples for QoS Guarantees • what if applications misbehave (audio sends higher than declared rate)? ‣ policing: force source adherence to bandwidth allocations • marking and policing at network edge: ‣ similar to ATM UNI (User Network Interface) 30 R1 R2 1 Mbps phone packet marking and policing provide protection (isolation) for one class from others Principle 2 CS 3251 - Computer Networks I Georgia TechPrinciples for QoS Guarantees • Allocating fixed (non-sharable) bandwidth to flow: inefficient use of bandwidth if flows doesn’t use its allocation 31 While providing isolation, it is desirable to use resources as efficiently as possible Principle 3 R1 R2 1.5 Mbps link 1 Mbps phone 1 Mbps logical link 0.5 Mbps logical link CS 3251 - Computer Networks I Georgia TechScheduling and Policy Mechanisms • scheduling: choose next packet to send on link • FIFO (first in first out) scheduling: send in order of arrival to queue ‣ real-world example? ‣ discard policy: if packet arrives to full queue: who to discard? • Tail drop: drop arriving packet • priority: drop/remove on priority basis • random: drop/remove randomly 32 CS 3251 - Computer Networks I Georgia TechScheduling Policies • Weighted Fair Queuing: ‣ generalized Round Robin ‣ each class gets weighted amount of service in each cycle ‣ real-world example? 35 CS 3251 - Computer Networks I Georgia TechPolicing Mechanisms • Goal: limit traffic to not exceed declared parameters • Three common-used criteria: ‣ (Long term) Average Rate: how many pkts can be sent per unit time (in the long run) • crucial question: what is the interval length: 100 packets per sec or 6000 packets per min have same average! ‣ Peak Rate: e.g., 6000 pkts per min. (ppm) avg.; 1500 ppm peak rate ‣ (Max.) Burst Size: max. number of pkts sent consecutively (with no intervening idle) 36 CS 3251 - Computer Networks I Georgia TechPolling Mechanisms • Token Bucket: limit input to specified Burst Size and Average Rate. • bucket can hold b tokens • tokens generated at rate r token/sec unless bucket full • over interval of length t: number of packets admitted less than or equal to (r t + b). 37 CS 3251 - Computer Networks I Georgia TechDiffserv Architecture • Edge router: ‣ per-flow traffic management ‣ marks packets as in-profile and out-profile • Core router: ‣ per class traffic management ‣ buffering and scheduling based on marking at edge ‣ preference given to in-profile packets 40 CS 3251 - Computer Networks I Georgia TechDiffserv Architecture • Edge router: ‣ per-flow traffic management ‣ marks packets as in-profile and out-profile • Core router: ‣ per class traffic management ‣ buffering and scheduling based on marking at edge ‣ preference given to in-profile packets 40 r b marking CS 3251 - Computer Networks I Georgia TechDiffserv Architecture • Edge router: ‣ per-flow traffic management ‣ marks packets as in-profile and out-profile • Core router: ‣ per class traffic management ‣ buffering and scheduling based on marking at edge ‣ preference given to in-profile packets 40 CS 3251 - Computer Networks I Georgia TechChapter 7 outline • 7.1 Multimedia Networking Applications • 7.2 Streaming stored audio and video • 7.3 Making the best out of best-effort service • 7.4 Protocols for Real-Time Interactive Applications ‣ RTP, ‣ RTCP, ‣ SIP • 7.5 Providing multiple classes of service • 7.6 Providing QoS Guarantees 42 CS 3251 - Computer Networks I Georgia TechNext Time • We will be reviewing for the final exam. ‣ There’s no reading to do for the lecture, but you may want to make sure that you’ve read for the exam. • Format, ground rules and an overview of subjects. ‣ Like before, I can and will draw from readings, lectures, discussions, homeworks and projects. ‣ I will also offer study tips to help get you through. • My exams are tough but fair - be prepared. 43